unit 9 Flashcards
What does ΔH stand for?
change in enthalpy
What does it mean if ΔH is negative?
reaction is exothermic; less energy than the start
What does it mean if ΔH is positive?
reaction in endothermic; more energy than the start
Are all combustion reactions endo or exo thermic?
exothermic (heat is given off; think of an explosion, it is not taking in heat, it is giving it off)
Definition of specific heat?
amount of heat required to raise one gram of the substance 1 degree Celsius
What is the symbol for specific heat?
c (joules/ grams C)
What is the symbol for energy/ heat?
q
What does it mean if q is positive?
energy is absorbed endothermic
What is another meaning for bond strength?
bond energy (use the bond energy equation)
If a reaction was positive enthalpy (+𐤃H or endothermic) and positive entropy (+𐤃S or more disorder), what is 𐤃G?
at high temperatures it will be thermodynamically favorable at low temperatures it will be thermodynamically unfavorable
If a reaction was negative enthalpy (-𐤃H or exothermic) and negative entropy (-𐤃S or more order), what is 𐤃G?
at low temperatures it will be thermodynamically favorable;at high temperatures it will be thermodynamically unfavorable
When a solid melts into a liquid is that an increase or decrease in entropy?
increase (more motion possible in a liquid)
When a solid is formed when two solutions are added together is that an increase or decrease in entropy?
decrease (a solid allows less motion/disorder of particles) (the opposite of this would be a solid dissolving/dissociating in a solution)
What does it mean if q is negative?
energy is lost (exothermic)
What is the enthalpy of formation for any element in its natural state?
0 (zero)
What does positive ΔS mean?
more disorder in the system
What does entropy mean?
randomness or disorder of a system
What does ΔS stand for?
entropy
What does negative ΔS mean?
less disorder in the system; more order
The enthalpy (𐤃H) for any element in its natural state is…
ZERO (ex. O2 has an enthalpy of zero when it is a gas)
What is the equation for bond energy?
sum of the energies of reactants - sum of energy for products
The entropy of a pure solid at absolute zero is…
Zero (you will rarely see this because we typically work in standard conditions)
Entropy is measured in…
Joules (mole-1) (Kelvin-1)
If 𐤃G is greater than zero what does it mean about the reaction?
It is NOT thermodynamically favorable (you can force the reaction but the products will turn right back into the reactants)
If 𐤃G is less than zero what does it mean about the reaction?
It IS thermodynamically favorable (the reaction will proceed in the forward direction)
What does it mean if 𐤃G is zero?
Reaction at equilibrium.
If a reaction was negative enthalpy (-𐤃H or exothermic) and positive entropy (+𐤃S or more disorder), what is 𐤃G?
NEGATIVE; the reaction will be thermodynamically favorable no matter what the tempurature
If a reaction was positive enthalpy (+𐤃H or endothermic) and negative entropy (-𐤃S or more order), what is 𐤃G?
POSITIVE; the reaction with be thermodynamically unfavorable no matter what the tempurature
When a solid or liquid becomes a gas is that an increase or decrease in entropy?
increase (more motion allowed in a gas than a solid or liquid)
When a gaseous chemical reaction produces more molecules is that an increase or decrease in entropy?
increase (there are even more randomly moving particles)
When the temperature of a substance decreases is that an increase or a decrease in entropy?
decrease (as energy decreases, the particles slow down and become more ordered) (the opposite of this is when temperature increases)
